The invention relates to supertwist liquid-crystal displays (SLCDs or supertwisted nematic (STN) displays) having very short response times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
SLCDs are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987), and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term SLCD here covers any relatively highly twisted display element with a value for the twist angle of between 160xc2x0 and 360xc2x0, such as, for example, the display elements of Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appi. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
SLCDs of this type are distinguished, in comparison to standard TN displays, by significantly better steepnesses of the electro-optical characteristic line and consequently better contrast values, and by significantly lower angle dependence of the contrast. Of particular interest are SLCDs having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures have hitherto been optimized using usually monotropic additives having relatively high vapour pressure. However, the response times achieved were not adequate for all applications.
In order to achieve a steep electro-optical characteristic line in SLCDs, the liquid-crystal mixtures should have relatively large values for K3/K1 and relatively small values for xcex94xcex5/xcex51.
In addition to optimization of the contrast and the response times, further important requirements are made of mixtures of this type:
1. A broad d/p window
2. High long-term chemical stability
3. High electrical resistance
4. Low frequency and temperature dependence of the threshold voltage.
The parameter combinations achieved are still far from adequate, in particular for high-multiplex, but also for low- and medium-multiplex STNs (1/400). This is in some cases attributable to the fact that the various requirements are affected in opposite manners by material parameters.
There thus continues to be a great demand for SLCDs, in particular for high-resolution displays (XGAs), having very short response times and at the same time a large operating temperature range, high characteristic line steepness, good angle dependence of the contrast and low threshold voltage which meet the abovementioned requirements.
The invention has an object of providing SLCDs which do not have the abovementioned disadvantages, or only do so to a lesser extent, and at the same time have very good response times, in particular at low temperatures, and very good steepnesses.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
Particularly, it has been found that advantages are achieved if nematic liquid-crystal mixtures are used which comprise compounds of the formula IA 
in combination with compounds of the formula IB 
in which
The use of the compounds of the formulae IA and IB in the mixtures for SLCDs according to the invention produces
high steepness of the electro-optical characteristic line
low temperature dependence of the threshold voltage and
very short response times, in particular at low temperatures.
The compounds of the formula IA are covered by the very broad generic claim in WO 95/30723 as components of liquid-crystalline media. However, the specific combination of the compounds of the formula IA with the compounds IB which produces, in particular, high steepness of the electro-optical characteristic line and low temperature dependence of the threshold voltage, is not described therein.
The compounds of the formulae IA and IB significantly shorten, in particular, the response times of SLCD mixtures while simultaneously increasing the steepness and retaining the low temperature dependence of the threshold voltage.
Furthermore, the mixtures according to the invention are distinguished by the following advantages:
they have low viscosity,
they have low temperature dependence of the threshold voltage and the operating voltage, and
they effect long storage times of the display at low temperatures.
The invention thus also includes to a liquid-crystal display having
two outer plates which, together with a frame, form a cell,
a nematic liquid-crystal mixture of positive dielectric anisotropy which is present in the cell,
electrode layers with alignment layers on the insides of the outer plates,
a pre-tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from about 0 degrees to 30 degrees, and
a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 22.5xc2x0 and 600xc2x0,
a nematic liquid-crystal mixture consisting of
a) 15-90% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 0-60% by weight of a liquid-crystalline component B consisting of one or more compounds having a dielectric anisotropy of between xe2x88x921.5 and +1.5;
c) 0-20% by weight of a liquid-crystalline component D consisting of one or more compounds having a dielectric anisotropy of below xe2x88x921.5, and
d) if desired, an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is from about 0.2 to 1.3,
characterized in that the liquid-crystal mixture additionally comprises at least one compound of the formula IA 
in which
and simultaneously comprises at least one compound of the formula IB 
in which
The invention also relates to corresponding liquid-crystal mixtures for use in SLCDs, in particular in medium- and low-multiplexed SLCDs.
The compounds of the formulae IA and IB are prepared by methods known per se, as described in the literature (Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), to be precise under reaction conditions which are known and suitable for said reactions.
Use can be made here of variants which are known per se, but are not mentioned here in greater detail.
The formula IB includes the following preferred compounds 
in which Rc, Y1 and p are as defined above, and Rc is an alkyl or alkoxy group having 1 to 7 carbon atoms. Rc is particularly preferably methyl, ethyl, n-propyl, n-pentyl, methoxy, ethoxy, n-propoxy or n-butoxy.
Further preferred compounds of the formula IB are those of the sub-formulae IB6 and IB7 
in which Rc and Re are as defined above, and Rf is alkenyl having 2 to 7 carbon atoms, in particular vinyl, 1E-propenyl, 1E-butenyl, 3E-butenyl, 1E-pentenyl or 3E-pentenyl.
Mixtures which, in addition to the compounds of the formula IA, comprise the compounds IB2, IB5, IB6 and/or IB7 are preferred.
Preferred compounds of the formulae IA and IB are those in which n and p, independently of one another, are 0 or 2.
Ra and Rc are preferably, independently of one another, H, a methyl group, an ethyl group or an n-propyl group, in particular H or a methyl group.
Rb is preferably a straight-chain alkoxy group having 1 to 7 carbon atoms. In particular, Rb is a methoxy, ethoxy or n-propoxy group. Rb is very particularly preferably a methoxy group.
Rd is preferably F, OCF3, OCHF2, a methyl, ethyl or n-propoxy group or a methoxy, ethoxy or n-propoxy group. Rd is particularly preferably F, OCF3 or OCHF2.
Preference is given to compounds of the formulae IB in which Y1 is F and Y2 is simultaneously H. Preference is furthermore given to compounds of the formula IB in which Y1 and Y2 are simultaneously H.
Of the compounds of the formulae IA and IB and the sub-formulae, preference is given to those in which at least one of the radicals present therein has one of the preferred meanings indicated.
The compounds of the formula IB1, IB2 and IB7 are particularly preferred.
Component A preferably comprises compounds of the formulae II and/or III 
in which
R is an alkyl, alkoxy or alkenyl group having 1 to 12 carbon atoms, in which, in addition, one or two non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COOxe2x80x94 in such a way that O atoms are not linked directly to one another, 
xe2x80x83are each, independently of one another 
where compounds of the formula IB are excluded from the scope of the formula III.
Preferred compounds of the formula II conform to the sub-formulae IIa to IIh: 
in which R, L1, L2 and L3 are as defmed above.
Particular preference is given to mixtures which comprise one or more compounds of the following sub-formulae 
in which R is as defined above.
Preference is furthermore given to mixtures which comprise one or more compounds of the formula IIh in which L2 is H and L1 is H or F, in particular F.
In a particularly preferred embodiment, component A additionally comprises compounds of the formulae AI to AIV: 
in which
R is an alkyl, alkoxy or alkenyl group having 1 to 12 carbon atoms, in which, in addition, one or two non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COOxe2x80x94 in such a way that O atoms are not linked directly to one another, 
xe2x80x83and Q1 are each, independently of one another, 
Z3 is 
xe2x80x83xe2x80x94CH2CH2xe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or a single bond.
The mixtures according to the invention preferably comprise one or more polar compounds having a high clearing point selected from the group consisting of the compounds AIV1 to AIV4: 
In the compounds AIV1 to AIV4, the 1,4-phenylene rings can also be laterally substituted by one or two fluorine atoms. Preferred compounds of this type are the compounds of the formulae AIV1-1, AIV1-2 and AIV1-3: 
In the mixtures according to the invention which comprise compounds of the formulae AIV1 to AIV4, the proportion of these compounds is preferably from about 2 to 25%.
Preferred compounds of the formula III conform to the sub-formulae IIIa-IIIv: 
in which R is as defined above, L3 and L4, independently of one another, are H or F, and R3 is alkyl or alkoxy having 1 to 7 carbon atoms.
Of the compounds of the formulae IIIa to IIIv, particular preference is given to those in which L4 is F, furthermore those in which L3 and L4 are F.
In addition to one or more compounds of the formulae IA and IB, preferred mixtures comprise one, two, three or more compounds of the formulae IIa, IIb, IIc, IIf, IIIb, IIId, IIIf, IIIh, IIIi, IIIm, IIIs, IIIt or IIIu, preferably one or more compounds of the formula IIIb, IIId, IIIh, IIIt or IIIu, and from one to four compounds of the formulae IA and IB and from one to three compounds of the formulae IIa, IIb and/or IIc.
In the above- and below-mentioned preferred compounds of the sub-formulae to the formulae II and III, R, R1 and R2, unless stated otherwise, are preferably straight-chain alkyl, alkenyl or alkoxy, in particular alkyl, having 1 to 12 carbon atoms, in particular having 1 to 7 carbon atoms.
Preference is furthermore given to mixtures which comprise one or more compounds of the sub-formula IIIb1 
in which R3 is as defined above.
In the compounds of the formula IIIb1, R3 is particularly preferably n-propyl, n-pentyl or n-heptyl.
The individual compounds, for example of the formulae II and III or their sub-formulae, or alternatively other compounds which can be used in the SLCDs according to the invention, are either known or can be prepared analogously to known compounds.
Preferred liquid-crystal mixtures comprise none or small amounts of one or more compounds of Component B, preferably from 2 to 20%. The compounds of group B are distinguished, in particular, by their low rotational viscosity values xcex31.
Further preferred liquid-crystal mixtures comprise a plurality of compounds of Component A, preferably from 20 to 65%, particularly preferably from 30 to 50%.
Component B preferably comprises one or more compounds selected from the group consisting of the compounds of the formulae IV1 to IV9: 
in which R1 and R2 are as defined for R, and R3 and R4, independently of one another, are an alkyl or alkoxy group having 1 to 7 carbon atoms.
Component B optionally additionally comprises one or more compounds selected from the group consisting of the compounds of the formulae IV10 to IV24: 
in which R1 and R2 are as defined for R, R3 and R4, independently of one another, are an alkyl or alkoxy group having 1 to 7 carbon atoms, and the 1,4-phenylene groups in IV10 to IV19, IV23 and IV24 may each, independently of one another, also be monosubstituted or polysubstituted by fluorine.
Particular preference is given to mixtures comprising one or more compounds of the following formula: 
in which R1# is alkenyl having 1 to 7 carbon atoms, and R2# is straight-chain alkyl having 1 to 4 carbon atoms.
In these compounds, R1# is particularly preferably vinyl, 1E-propenyl, 1-butenyl, 3E-butenyl or 3E-pentenyl. R2# is particularly preferably methyl, ethyl or propyl, in particular methyl or ethyl.
Component B furthermore preferably comprises one or more compounds selected from the group consisting of the compounds of the formulae IV25 to IV31: 
in which R1 and R2 are as defined for R, and L is F or H. The 1,4-phenylene groups in the compounds IV25 to IV31 may also each, independently of one another, be monosubstituted or polysubstituted by fluorine.
Mixtures which comprise compounds of the formula IV25 in which L is F are preferred.
Particular preference is given to compounds of the formulae IV25 to IV31 in which R1 is alkyl and R2 is alkyl or alkoxy, in particular alkoxy, in each case having 1 to 7 carbon atoms. Preference is furthermore given to compounds of the formulae IV25 and IV31 in which L is F.
In the compounds of the formulae IV1 to IV15 and IV17 to IV31, R1 and R2 are particularly preferably straight-chain alkyl or alkoxy having 1 to 12 carbon atoms.
Component B optionally comprises one or more compounds selected from the group consisting of the compounds of the formulae VI and VII: 
in which CrH2r+1 is a straight-chain alkyl group having up to 9 carbon atoms.
In a further preferred embodiment, the liquid-crystal mixtures according to the invention, besides components A, B, C and D, additionally comprise one or more compounds selected from the group consisting of the compounds of the formulae VIII and IX 
in which R1 and R2 are as defined above.
Preference is furthermore given to liquid-crystal mixtures comprising at least one component selected from the group consisting of the compounds of the formulae X to XIV: 
in which Hal is F or Cl, L is H or F, and R and R3 are as defined above, in particular in which R and R3 are alkyl having 1 to 5 carbon atoms.
The liquid-crystalline mixtures optionally comprise an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is greater than 0.2. For the component, a multiplicity of chiral dopants, some of which are commercially available, is available to the person skilled in the art, for example such as cholesteryl nonanoate, S-811 from Merck KGaA, Darmstadt, and CB15 (BDH, Poole, UK). The choice of dopants is not crucial per se.
The proportion of the compounds of component C is preferably from 0 to 10%, in particular from 0 to 5%, particularly preferably from 0 to 3%
In a particularly preferred embodiment, the mixtures according to the invention comprise from about 2 to 45%, in particular from 5 to 25%, of liquid-crystalline tolan compounds. This enables smaller layer thicknesses to be used, significantly shortening the response times. The tolan compounds are preferably selected from Group T consisting of the compounds of the formulae T1, T2 and T3: 
in which 
preferably 
also 
in the formula T1 and also 
in the formula T2,
Preferred compounds of the formula T1 conform to the sub-formulae T1a and T1b 
in which L1 to L4 are H or F, and Qxe2x80x94Y is F, Cl or OCF3, in particular F or OCF3.
Preferred compounds of the formula T2 conform to the sub-formulae T2a to T2g 
in which R1, R2 and Z4 are as defined above, and L1 to L6 are H or F.
Particularly preferred compounds of the formula T2e are those in which one, two or three of the radicals L1 to L6 are F and the others are H, where L1 and L2 or L3 and L4 or L5 and L6 are not both simultaneously F.
Preferred compounds of the formula T3 conform to the sub-formulae T3a to T3e 
in which R1, R2 and Z4 are as defined above.
The proportion of compounds from Group T is preferably from 2 to 45%, in particular from 5 to 30%.
In a further particularly preferred embodiment, the mixtures according to the invention preferably comprise from about 5 to 20% of one or more compounds having a dielectric anisotropy of less than xe2x88x922 (component D).
Component) preferably comprises one or more compounds containing the structural unit 2,3-difluoro-1,4-phenylene, for example compounds as described in DE-A 38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Particular preference is given to tolans containing this structural unit, as described in International Patent Application PCT/DE 88/00133, in particular those of the formulae T2f and T2g.
Further known compounds of component D are, for example, derivatives of 2,3-dicyanohydroquinones or cyclohexane derivatives containing the structural unit 
as described in DE-A 32 31 707 or DE-A 34 07 013 respectively.
The liquid-crystal mixture according to the invention preferably comprises one or more compounds selected from Group 1 consisting of compounds of the formulae B1I to B1IV: 
in which
R1, R2 and Z4 are as defined above and 
and/or at least one compound selected from Group B2 consisting of compounds of the formulae B2I to B2III: 
in which
and/or at least one compound selected from Group B3 consisting of compounds of the formulae B3I to B3III: 
in which
The proportion of the compounds from Group B1 is preferably from 10 to 50%, in particular from 15 to 40%. Compounds of the formulae B1III and B1IIV are preferred.
Particularly preferred compounds from Group B1 are those of the following sub-formulae: 
in which
The proportion of the compounds of the abovementioned sub-formulae B1IIIa and B1IIIb together with the compounds of the formula IB1 is preferably from about 5 to 45%, particularly preferably from about 10% to 35%.
The proportion of the compounds of the sub-formula B1IVa or of the compounds of the formula B1IV is preferably from about 5 to 40%, particularly preferably from about 10 to 35%.
In a particularly preferred embodiment, the mixtures simultaneously comprise compounds of the formulae B1III and B1IV together with the compounds of the formulae IB1 and IB2, where the total proportion for components from Group B1 is observed.
If compounds of the formulae B1I and/or B1III are present, R1 and R2 are preferably each, independently of one another, n-alkyl having 1 to 7 carbon atoms or (trans)-n-alkenyl having 3 to 7 carbon atoms. Z is preferably a single bond.
Preference is furthermore given to mixtures according to the invention which comprise one or more compounds of the formula B1IV in which 
is 
and R1 and R2 have one of the preferred meanings indicated above and are particularly preferably n-alkyl having 1 to 7 carbon atoms.
In all cases, the total proportion of components from Group B1 is observed.
The proportion of the compounds from group B2 is preferably from about 0 to 45%, in particular from 5 to 20%. The proportion (preferred ranges) for B2I to B2III is as follows:
Preferred compounds from Group B2 are shown below: 
in which R1, R2, L and Z5 are as defined above.
In these compounds, R1 is preferably n-alkyl having 1 to 7 carbon atoms or (trans)-n-alkenyl having 3 to 7 carbon atoms, Z5 is preferably a single bond, R2 preferably has the preferred meaning given above for R or is fluorine, and L is preferably fluorine.
The mixtures according to the invention preferably comprise one or more compounds selected from the group consisting of B2Ic, B2IIa and B2IIIa in a total proportion of from about 5 to 35%
In a particularly preferred embodiment, the mixtures according to the invention, in addition to B2Ic, B2IIa and B2IIIa (Lxe2x95x90F), comprise further terminally fluorinated compounds, selected, for example, from the group consisting of 
and/or polar heterocyclic compounds selected from the group consisting of 
in which R1 is preferably n-alkyl having 1 to 7 carbon atoms or (trans)-n-alkenyl having 3 to 7 carbon atoms, h is 0 or 1, X0 is F, Cl, CF3, xe2x80x94OCF3 or xe2x80x94OCHF2, and L is H or F.
The total proportion of all terminally fluorinated compounds is preferably from about 5 to 75%, in particular from about 15 to 50%.
The proportion of compounds from Group B3 is preferably from about 5 to 30%, particularly preferably from about 10 to 20%. R1 is preferably n-alkyl or n-alkoxy, in each case having 1 to 9 carbon atoms.
However, it is also possible to employ analogous compounds containing alkenyl or alkenyloxy groups. Compounds of the formula B3I are preferred.
The term xe2x80x9calkenylxe2x80x9d in the definition of R, R1, R2, Rf and Rd covers straight-chain and branched alkenyl groups having carbon up to 7 atoms, in particular the straight-chain groups. Particularly preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl.
Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
The terms xe2x80x9calkylxe2x80x9d and xe2x80x9calkoxyxe2x80x9d in the definitions of Ra, Rb, Rc, Rd, Re, R, R1, R2, R3 and R4 cover straight-chain and branched alkyl and alkoxy groups, in particular the straight-chain groups. Particularly preferred alkyl and alkoxy groups are ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexyloxy or heptyloxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, methoxy, octyloxy, nonyloxy, decyloxy, undecyloxy or dodecyloxy.
The mixtures according to the invention comprise compounds of the formulae IA and IB and preferably compounds from at least one of groups B1, B2 and B3. They preferably comprise one or more compounds from Group B1 and one or more compounds from Group B2 and/or B3.
In a preferred embodiment, the liquid-crystalline media according to the invention comprise 3, 4, 5 or 6 compounds of the formulae IA and/or IB; the content of these compounds is generally from 10 to 80% by weight, preferably from 15 to 50% by weight, based on the mixture as a whole.
In a further preferred embodiment, the mixtures comprise
one or more compounds of the following formulae 
in which R1, R2 and L have the preferred meanings given under compounds of Component B. The proportion of these compounds in the liquid-crystal mixtures is preferably from 0 to 45%, in particular from 5 to 30%;
one or more, in particular 1, 2, 3 or 4, compounds selected from the compounds of the formulae IIIb, IIId, IIIf, IIIh, IIIi, IIIm, IIIs, IIIt and IIIu;
at least two compounds selected from the compounds of the formulae IIb1, IIb2, IIc1 and IIc2. The proportion of these compounds in the liquid-crystal mixtures is preferably from 0 to 60% by weight, particularly from 10 to 45%;
one or more compounds of the formula T1 or T2, in particular one or more compounds of the formula T2a and/or T2b, where the proportion of these compounds in the liquid-crystal mixtures is preferably from 0 to 25%, in particular from 1 to 15%.
Further particularly preferred embodiments relate to liquid-crystal mixtures comprising
at least two compounds of the formula AI or AII;
one or more compounds in which R is a trans-alkenyl group or trans-alkenyloxy group;
one or more compounds selected from the following group: 
in which R1, R2 and L have the preferred meanings given under compounds of Component B, and R3 and R4 are as defined above. The 1,4-phenylene group in the abovementioned compounds may also be substituted by fluorine;
one or more compounds of the formulae 
in which R, R1 and R2 are as defined above.
In particular when used in SLCDs having high layer thicknesses, the mixtures according to the invention are distinguished by very low total response times (ttot=ton+toff). Low total response times are an important criterion, in particular, for SLCDs for use as displays in laptops in order to be able to display cursor movements without interference.
The liquid-crystal mixtures used in the STN and TN cells according to the invention are dielectrically positive with xcex94∈xe2x89xa71. Particular preference is given to liquid-crystal mixtures where xcex94∈xe2x89xa73 and very particularly to those where xcex94∈xe2x89xa75.
The liquid-crystal mixtures according to the invention have favourable values for the threshold voltage V10/0/20 and for the rotational viscosity xcex31. If the value for the optical path difference dxc2x7xcex94n is specified, the value for the layer thickness d is determined by the optical anisotropy xcex94n. In particular at relatively high values for dxc2x7xcex94n, the use of liquid-crystal mixtures according to the invention having a relatively high value for the optical anisotropy is generally preferred since the value for d can then be chosen to be relatively small, which results in more favourable values for the response times. However, liquid-crystal displays according to the invention which contain liquid-crystal mixtures according to the invention having relatively small values for xcex94n are also characterized by advantageous values for the response times.
The liquid-crystal mixtures according to the invention are furthermore characterized by advantageous values for the steepness of the electro-optical characteristic line and can be operated at high multiplex rates, in particular at temperatures above 20xc2x0 C. In addition, the liquid-crystal mixtures according to the invention have high stability and favourable values for the electrical resistance and the frequency dependence of the threshold voltage. The liquid-crystal displays according to the invention have a broad operating temperature range and good angle dependence of the contrast.
The construction of the liquid-crystal display elements according to the invention from polarizers, electrode base plates and electrodes having a surface treatment such that the preferential alignment (director) of the liquid-crystal molecules in each case adjacent thereto is usually twisted by a value of from 160xc2x0 to 720xc2x0 from one electrode to the other corresponds to the structure which is conventional for display elements of this type. The term xe2x80x9cconventional structurexe2x80x9d is broadly drawn here and also includes all derivatives and modifications of the TN and STN cell, in particular also matrix display elements, and display elements which contain additional magnets.
The surface tilt angle at the two outer plates may be identical or different. Identical tilt angles are preferred. Preferred TN displays have pre-tilt angles between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0xc2x0 to 7xc2x0, preferably from 0.01xc2x0 to 5xc2x0, in particular from 0.1 to 2xc2x0. In STN displays, the pre-tilt angle is from 1xc2x0 to 30xc2x0, preferably from 1xc2x0 to 12xc2x0, in particular from 3xc2x0 to 10xc2x0.
The twist angle of the TN mixture in the cell has a value of between 22.5xc2x0 and 170xc2x0, preferably between 45xc2x0 and 130xc2x0, in particular between 80xc2x0 and 115xc2x0. The twist angle of the STN mixture in the display from alignment layer to alignment layer has a value of between 100xc2x0 and 600xc2x0, preferably between 170xc2x0 and 300xc2x0, in particular between 180xc2x0 and 270xc2x0.
The liquid-crystal mixtures which can be used according to the invention are prepared in a manner which is conventional per se. In general, the desired amount of the components used in a lesser amount is dissolved in the components making up the principal constituent, advantageously at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and, after mixing, to remove the solvent again, for example by distillation.
The dielectrics may also contain further additives which are known to the person skilled in the art and are described in the literature. For example, 0-15% of pleochroic dyes may be added.
The entire disclosure of all applications, patents and publications, cited above, and of corresponding German application No. 199 20 405.5, filed May 4, 1999 is hereby incorporated by reference.